Thread-forming, self-tapping screws are known in the art for a variety of uses and applications. Unlike a standard screw that forms the male component which accepts a nut, or other female component of a threaded fastener system, a thread forming screw is driven into a body, forming threads therein as the screw advances. Depending on the size of the screw, the composition of the body in which it is driven and other factors, a pilot hole may be provided in the body before the screw is inserted.
One form of thread-forming screw, commonly referred to as a sheet metal screw, is provided for fastening together two relatively thin panels of material. Sheet metal screws have many applications and uses, including assembling panels and components of appliances, securing sections of ductwork, securing small parts of assemblies of various types, and the like. Some difficulties encountered in using sheet metal screws include providing sufficient holding force (resistance to stripout) with a lower driving force in relatively thin, sheet-like materials. Since the panels held together frequently are thin, a screw that is inserted through the panels often will protrude beyond the furthest surface from the first surface through which the screw is inserted. Screw ends protruding in this manner can damage materials and things coming in contact therewith, such as wires in appliances. The sharp, protruding points of known sheet metal screws can be a source of personal injury in the way of scratches and puncture wounds to persons coming in contact therewith.
Problems encountered in the design of sheet metal screws increase as the materials for which the screws are used become thinner, from efforts to reduce weight and decrease costs in the final assemblies, such as appliances. Thus, it is known to provide sheet metal screws of one type or design for use in very thin materials and of a different type or design for use in somewhat thicker materials. Even so, strip-out remains a problem in thinner materials. It also is known that one screw will insert readily and quickly while a next similar screw used in the same material will not “start” readily, but instead will dwell in the pilot hole with the threads riding on the surface of the material without advancing into the material. It is cumbersome and disadvantageous to have to obtain, store and use a variety of different sheet metal screws, depending upon the thickness of the material in which it is used. A single screw design useful in both thin and thick materials provides advantages to both the manufacturer and the user of the screw. Particularly on assembly lines, it is desirable that attachment sequences be performed consistently, and that all similar sheet metal screws inserted into similar materials react in the same way in starting, driving and seating in the material.
It has been a long-accepted standard of the fastener art to provide sheet metal screws in standard sizes based on thickness. Numerical designations have been used for the different sizes, with the larger number being a thicker screw. Thus, by way of example, a #10 screw is thicker than a #8 screw, which is thicker than a #6 screw, which is thicker than a #4 screw. It is also well known and universally accepted among screw manufacturers and users that different screw sizes are provided with different numbers of threads per inch of screw length. Again by way of example, it is commonly known and understood that a #4 screw has 24 threads per inch of shaft length, a #8 screw has 18 threads per inch and a #10 screw has 16 threads per inch. This increases the complexity of manufacturing facilities and equipment needed to manufacture sheet metal screws in a variety of different sizes.
What is needed in the art is a standardized thread-forming tapping screw that overcomes some or all of the aforementioned disadvantages of known thread-forming tapping screws.